386 A Manual of Veterinary Physiology. 



the horns of the uterus. The urachus communicates with 

 the allantois as in the mare ; but, unlike the latter, it is 

 constantly found to be pervious at birth. 



Surrounding both these sacs we have the Chorion, which 

 forms the connection, through the medium of the umbilical 

 cord, between the mother and embryo. It is developed 

 from the outer or vitelline covering of the embryo, and 

 important differences are observable between its arrange- 

 ment in the mare and in ruminants. In the former the 

 chorion is attached all over the inner surface of the uterus, 

 excepting at the os ; in ruminants the chorion is attached 

 to the uterus by cotyledons, dotted here and there over the 

 surface, about 60 or 80 in number ; while in the pig the 

 placenta is like that of the mare. 



The development of the chorion from the vitelline layer 

 occurs by tufts of vessels forming on its exterior, either all 

 over as in the mare, or dotted here and there as in the 

 cow. These villi project themselves into the mucous mem- 

 brane of the uterus, not through the medium of a decidua, 

 as in the woman, but directly into the wall of the uterus, as 

 solipeds and ruminants possess no decidua. As the um- 

 bilical cord forms the connection between the embryo and 

 the chorion, a perfect communication is kept up between 

 the mother and embryo, not by the direct passage of blood 

 from mother to foetus, but through the capillary Avails only, 

 for the vessels of the one do not directly communicate with 

 the vessels of the other. 



The umbilical cord is composed of two portions : an 

 amniotic, which is nearest the foetus, and an allantoic, 

 which is next the chorion and outside the amniotic cavity j 

 this is the arrangement in the mare. The umbilical cord 

 in ruminants has no fold of allantois, owing to the anatomical 

 difference in the arrangement of this sac, but it directly 

 enters the chorion from the amnion. In both classes of 

 animal the cord is made up of a gelatinous material, known 

 as ' Wharton's jelly,' containing the umbilical arteries and 

 veins passing to and from the chorion. The cord is about 



